1. Field of the Invention
The present invention relates to a method of making individual castings, for example in the field of dental engineering or jewelry production.
2. Description of the Prior Art
To make prosthetic metal structures in dental engineering, e.g. crowns, inlays or bridges, up to now the latter are first modeled in wax, as a result of which they are provided with the appropriate individual shape. A prefabricated sprue is secured to this so-called casting pattern or model; thereafter, the casting model is enveloped in a refractory embedding compound. The wax composition is then removed either by boiling out with hot water, or by burning the wax at temperatures between about 500.degree. and 700.degree. C. Molten metal is then introduced into the resulting negative mold, is allowed to cool, and is removed from the mold, so that a casting having the same shape as the wax casting model is thus obtained.
It is obvious and readily apparent that with regard to its accuracy of fit in the corresponding situation in the mouth, the casting is only as good as the wax casting model. Generally, the dental technician has a jaw model, for example of plaster, to which he adapts the casting model. However, when the parts modeled in wax are removed from the model, there is a great danger that the casting model becomes distorted and that therefore the metal casting finally obtained does not fit. Moreover, during the modeling, the wax must be liquefied by heating, so that, particularly with large bridge structures, strong thermal stresses arise which can lead to a distortion or breaking of the casting model. Also, due to nonuniform temperatures, different wax consistencies are obtained, so that the uniformity of the wax application is not always ensured, and the modeling as a whole is made difficult. An additional danger is the formation of streaks and bubbles.
To avoid deformation of the wax when it is removed from the model, a procedure has been adopted in which caps of plastic deep-draw foil are made over crown stumps. However, there is again a danger of deformation due to the subsequent thermal stress during the application of the molten wax. The situation is particularly difficult in the case of multi-part removable structures, e.g. conical crowns, telescopic crowns, or double crowns for interlocking. These structures consist fundamentally of a metal crown, which is referred to as the primary part and is securable to the tooth stump, and over which a so-called secondary crown is made in exact fit. Prostheses or bridges are subsequently secured to this secondary part, and are thus removable. A requirement for the fulfillment of the holding function of secondary parts on primary parts is an exactly fitting production of the secondary crown on the primary crown. The same applies to attachment, lock and bar structures. Since it is very difficult to model these interengaging parts from wax, to increase the stability particularly the secondary parts of these detachable prostheses are modeled with self-polymerizing methyl methacrylate, at least to a certain layer thickness. Such a procedure is described in "Quintessenz" 1965, No. 6, pages 57 and 58.
These compounds, which usually cure on the principle of redox polymerization, have, however, a relatively short working time, yet require a relatively long time until they are completely polymerized. Due to the high temperature peak during the curing, thermal stresses easily occur; in addition, bubbles can form easily at the surface. There is further necessary, because of the high polymerization shrinkage, to apply great amounts of the compound, and to grind it back to the final size. This fact prevents an exact anatomical modeling in the build-up method, and therefore makes it extremely complicated and time-consuming. This is also because the polymerization starts immediately after the mixing of the compound, and the consistency of the compound changes continuously during the work. At the start of the work, the methyl methacrylate preparation remains liquid for a relatively long time, so that the parts which are not to be covered must be protected from overflow in complicated manner, e.g. by a wax ring, from overflow. Working with these polymers is thus a very complicated and involved procedure which moreover results in excessively high material consumption.
An object of the present invention therefore is to provide a method which permits the simple production of exactly fitting casting models, even in the case of very difficult structures, in particular in dental engineering.
This object, and other objects and advantages of the present invention, will appear more clearly from the following specification in conjunction with the accompanying Examples.